While a number of biodegradable polymers have been found to possess the desirable characteristics of biodegradability and compostability, they often lack additional properties that are desired or necessary to provide more commercially acceptable products. At room temperature many biodegradable polymers are either too brittle to provide the desired puncture and tear resistance necessary for many applications, or they do not have adequate stability for storage and transport. In addition, many biodegradable polymers are difficult to process into films using commercial manufacturing lines.
In attempts to overcome such difficulties, blends of polymeric materials with other polymers or with naturally biodegradable components have been attempted in efforts to develop thermoplastic films with improved degradable properties. For example, U.S. Pat. No. 4,133,784 to Otey et al. describes degradable mulch films with improved moisture resistance prepared from starch and ethylene/acrylic acid copolymers. U.S. Pat. No. 5,091,262 to Knott et al. describes a multilayer polyethylene film containing a starch filled inner layer, and prodegradant filled outer layers. U.S. Pat. No. 5,108,807 to Tucker describes a multilayer thermoplastic film having a core layer made of polyvinyl alcohol, and outer layers made of polyethylene and prodegradant. U.S. Pat. No. 5,391,423 to Wnuk et al. describes multilayer films prepared from various biodegradable polymers for use in disposable absorbent products, such as diapers, incontinent pads, sanitary napkins, and pantyliners.
Typical, non-degradable or slowly degradable plastic products in the form of sheets and films (e.g., as in plastic trash bags and package wrapping materials) are made primarily from hydrocarbon polymers such as polyethylene, polypropylene, or polyvinyl polymers. The combination of such hydrocarbon polymers with starch has not been very widely accepted. For example, trash bags incorporating starch with other hydrocarbon components can be physically degradable, which means they are broken into many small parts as the starch biodegrades. See, for example, U.S. Pat. No. 4,016,117 to Griffin, and U.S. Pat. No. 4,337,181 to Otey et al. See also Pettijohn (1992), “Starch/Polyolefin Blends as Environmentally Degradable Plastics,” Chemtech, 627; Willett (1994) J. Appl. Polym. Sci. 54:1685-1695. Initially, the starch particles exposed at, or adjacent to the surface of these starch-containing products, are initially biodegraded and leached away. This is followed by successive biodegradation of starch particles at the interior of the product to provide a cellular structure that is more readily attacked by the processes of oxidation, hydrolysis, direct enzyme action or combinations of these processes. However, such starch-containing products still leave behind a non-biodegradable polymer residue as recognized in the art, for example by U.S. Pat. No. 5,219,646 to Gallagher et al. The hydrocarbon components remain resistant to degradation and to mineralization. In certain circumstances, it is believed that the hydrocarbon component even has a tendency to encapsulate the starch and thereby preventing further biodegradation of the starch. Furthermore, materials incorporating large amounts of starch can be very sensitive to moisture and can have mechanical properties that vary considerably with humidity levels. Accordingly, improved polymeric compositions for making better biodegradable films are needed.
There is also a need for degradable fibers that can be widely used without polluting the environment. Such improved fibers are needed as fishing materials, such as fishing lines and fish nets; in agricultural materials such as insect or bird nets and as vegetation nets; in cloth fibers and non-woven fibers for articles for everyday life; in personal care products such as diapers, incontinence pads, sanitary napkins, pantyliners, tampons, and diapers; and in medical supplies such as operating sutures that are not removed, operating nets and suture-reinforcing materials. Fibers that are degradable by the action of microorganisms have been described. Examples of such fibers comprising lactones or polyester fibers are described in U.S. Pat. No. 6,235,393 to Kimura et al. However, such fibers are difficult and/or expensive to manufacture while maintaining quality control, and some products are difficult to use due to insufficient flexibility. A recently popular form of fiber made from synthetic polymers is referred to as “bicomponent” fibers. A bicomponent fiber comprises a core fiber made from one polymer that is encased within a thermoplastic sheath made from a different polymer. The polymer comprising the sheath often melts at a different, typically lower, temperature than the polymer comprising the core. As a result, such bicomponent fibers can provide thermal bonding by controlled melting of the sheath polymer, while retaining the desirable strength characteristics of the core polymer. An outer sheath is typically comprised of polyethylene, polypropylene, certain polyesters, and the like, that often have softening and/or melting points in the range of about 50° C. to about 200° C. Generally, however, such fibers are still difficult and/or expensive to manufacture.
There are a number of other polymer-based products for which degradability and/or compostability would be desirable. For example, films and laminates that are used in packaging materials, as topsheets and backsheets in diapers, and as agricultural ground coverings are intended to survive intact for only a short period of use. Other polymer-based products for which degradability is desirable are molded articles such as tampon applicators, disposable syringes, milk bottles, shopping bags, food wrappers, beverage “six-pack” rings, and the like. Ideally, such molded articles would be substantially degraded in the sewage system or septic tank, or would decompose at the site of disposal so as to avoid causing visual litter problems or hazards to wildlife.
Plastic film products for agricultural mulching are representative of the problems that can be caused by the persistence of synthetic polymers. Polyethylene is the most common polymer used in making agricultural mulch products, and blends with starch have similar drawbacks to those described above for trash bag containers. Like flexible film products for packaging and garbage bags, such agricultural mulch products can persist for many years and become a nuisance. There is a need for plastic mulch products that can decompose by the end of a growing season. Improved degradability would also be desirable to provide items for “controlled release” of active agents from other agricultural products, such as encapsulated pesticides, herbicides, and fertilizers.
Several approaches to enhance the environmental degradability of polymers have been suggested and tried. Photosensitizing groups have been added into the molecular structure of the polymer, and small amounts of selective additives have been incorporated to accelerate oxidative and/or photo-oxidative degradation. However, photodegradation only works well if the plastic is exposed to light, and provides no benefit if the product is disposed of in a dark environment such as ground water, soil or a standard landfill. Also, oxidative accelerators can cause unwanted changes in the mechanical properties of the polymer, such as embrittlement, prior to or during use.
Another approach to environmental degradability of articles made with synthetic polymers is to make the polymer itself biodegradable or compostable. See Swift (1993) Acc. Chem. Res. 26:105-110 for a general overview on biodegradable polymeric compositions. Most of this work has been based on hydrolyzable polyester compositions, chemically modified natural polymers such as cellulose or starch or chitin, certain polyamides, or blends thereof. See, for example, U.S. Pat. No. 5,219,646 to Gallagher et al.(blend of hydrolyzable polyester and starch). Polyvinyl alcohol is the only synthetic high molecular weight addition polymer with no heteroatom in the main chain generally acknowledged as biodegradable, but consistent polymeric production is difficult. See also Hocking (1992) J. Mat. Sci. Rev. Macromol. Chem. Phys. C32(1): 35-54, Cassidy et al. (1981) J. Macromol. Sci.—Rev. Macromol. Chem. C21(1):89-133, and “Encyclopedia of Polymer Science and Engineering,” 2nd. ed.; Wiley & Sons: New York, 1989; Vol. 2, p220. (Limited reports add poly (alkyl 2-cyanoacrylates) to this list of biodegradable synthetic polymers.)
Natural rubber (cis-1,4-polyisoprene) is also readily biodegradable. Natural rubber retains carbon-carbon double bonds in the polymer backbone, which are believed to facilitate attack by either oxygen and/or microbes/fungi, leading subsequently to chain scission, molecular weight reduction, and eventually total degradation of the polymer. See Heap et al. (1968) J. Appl. Chem. 18:189-194. The precise mechanism for the biodegradation of natural rubber is not known. Enzymatic and/or aerobic oxidation of the allylic methyl substituent may be involved. See Tsuchii et al. (1990) Appl. Env. Micro., 269-274, Tsuchii et al. (1979) Agric. Biol. Chem. 43(12): 2441-2446, and Heap et al., supra. By contrast, nonbiodegradable polymers such as polyethylene, polypropylene, polyvinyl chloride, polyacrylonitrile, poly(meth)acrylates and polystyrene have saturated carbon-carbon backbones that do not facilitate attack by either oxygen and/or microbes. This biodegradability has been recognized only for the natural form of rubber.
Unfortunately, natural rubber is biodegradable to the extent that it is too unstable for most uses. Natural rubber also suffers from poor mechanical properties (e.g., strength, creep resistance). Indeed, stabilizers, fillers, and/or crosslinking agents are routinely added to natural rubber to enhance its mechanical properties. Crosslinkers are typically required in order to provide sufficient mechanical integrity for practical use. However, the most common crosslinking process creates a polysulfide linkage, i.e., by vulcanization, that virtually eliminates the biodegradability of natural rubber. See Tsuchii et al. (1990) J. Appl. Polym. Sci. 41:1181-1187. Crosslinked natural rubber is also elastomeric and thermosetting, thus making it unsuitable for blown or extruded films, injection molded articles, fibers or other melt-processed articles.
It would be desirable to provide polymer-containing products that: (1) are biodegradable in the environment, as well as degradable or compostable during municipal waste handling operations; (2) are thermoplastic so that they can be molded, cast, extruded, or otherwise melt-processed into various forms including films, fibers, coatings, foams, and the like; (3) can be manufactured at reasonable cost; and (4) have sufficient toughness, strength and stability during use until they are appropriately disposed of. Therefore, polymers or copolymers are needed that provide reproducible and predictable properties with respect to degradation and environmental hydrolysis, and that hydrolyze to a very significant extent to provide small, soluble, and generally nontoxic polymer fragments.
For many purposes, the superior physical properties provided by polyolefins prepared by addition polymerization are desirable. To date, however, the incorporation of polar moieties, e.g., hydrolyzable polar linkages, into such polymers has had limited success, since many polar monomers poison, or competitively coordinate with, the organometallic polymerization catalysts that are typically used. Copolymers of olefins, such as ethylene, with polar monomers such as acrylates, were initially limited to block copolymers, formed by two-stage polymerization, e.g., by post-polymerization of an acrylate or methacylrate monomer onto a previously formed polyolefin chain. See U.S. Pat. No. 5,563,219 to Yasuda et al., EP 0799842 to Yasuda et al., EP 0462588 to Goto et al., and EP 0442476 to Hajime et al. JP Kokai 4-45108 pertains to the preparation of an ethylene copolymer containing 4.7 mole % ethyl acrylate (number average molecular weight Mn of 9,100, weight average molecular weight Mw of 22,500) that is described as exhibiting improved adhesion relative to homopolymeric polyethylene. Johnson et al. (1996), J. Am. Chem. Soc. 118:267-8, described the formation of random olefin-acrylate copolymers using Brookhart-type catalysts. None of these polymers, however, include hydrolyzable linkages in the backbone of the polymer, and therefore they would not be hydrolytically degradable.
Ouchi et al. (1968), J. Chem. Soc. Japan 71(7):1078-82, described free radical copolymerization of styrene and other vinyl monomers with a monomer containing a hydrolyzable linkage, diallylidene pentaerythritol (DAPE). However, the process resulted in a copolymer (1) in which relatively little hydrolyzable monomer was incorporated, or (2) exhibiting a significant loss in intrinsic viscosity at higher levels of incorporation. Higher levels of hydrolyzable monomer incorporation were also found to be associated with a lower polymerization rate. Additionally, the reaction conditions employed would be expected to result in a non-stereoregular polymer.
Austin et al., in International Patent Publication No. WO 92/12185, describes a method for making biodegradable and photodegradable polymers containing ester linkages. The disclosed polymerization method involves a radical-initiated ring-opening copolymerization reaction between ethylene and a cyclic ketene acetal, 2-methylene-1,3-dioxepane (MDOP). The resulting copolymer contains both ethylene monomer units (—CH2—CH2—) and ester-containing monomer units having the structure —(CO)—O—(CH2)4—. The maximum amount of the ester-containing monomer units incorporated into the copolymer, however, was only 3.20 mole %, even when the amount of MDOP in the feed was increased to 25 wt. %. Such a copolymer would hydrolyze to a very limited extent and be of minimal utility in providing degradable products.